Nearly five percent of Americans now suffer from extreme obesity, defined by a body mass index (BMI) >= 40 kg/m 2. This condition is associated with dire health and economic consequences that make weight reduction imperative. Bariatric surgery is the most effective intervention for extreme obesity and will be sought this year by approximately 125,000 individuals. The gastric bypass (GBP) reduces initial body weight by an average of 25 percent to 30 percent;however, treatment outcome is variable. A substantial minority of individuals fails to achieve significant weight loss, and even more regain weight two or more years after surgery. As a result, leading bariatric surgeons recommend that surgical candidates undergo a behavioral (psychiatric) evaluation to identify those with psychosocial contraindications. There are few data, however, to inform patient selection or the need for postoperative counseling. Binge eating disorder (BED) is common among extremely obese individuals and is arguably the most likely behavioral complication to affect the outcome of bariatric surgery. BED is characterized by the consumption of an objectively large amount of food and the experience of loss of control during overeating episodes. It also is associated with increased symptoms of depression and other psychopathology. The proposed research will address critical gaps in our knowledge concerning whether binge eating disorder negatively affects the outcome of GBP, either by limiting weight loss or by increasing the rate of postoperative behavioral complications (including vomiting and dumping). These findings should guide patient selection and the provision of postoperative counseling. The study, in turn, will reveal the effects of GBP on binge eating, energy and macronutrient intake, appetite, and physical activity. Changes over 30 months in patients with (N = 80) and without BED (N = 80) will be compared with each other but also with changes in a group of comparably obese individuals with BED who are treated by a traditional behavioral weight control program. Inclusion of this latter group (N = 50) will more fully reveal the behavioral risks and benefits of GBP. The study, by examining ghrelin and other appetite-related hormones, also will enhance knowledge of the regulation of body weight following GBP.